test: add comprehensive tests for all Epic 1 stories

Story 1.2: Database Layer
- Test database client creation, connection, ping, and close
- Test connection pooling configuration
- Tests skip if database is not available (short mode)

Story 1.3: Health Monitoring and Metrics
- Test health registry registration and checking
- Test database health checker
- Test liveness and readiness checks
- Test metrics creation, middleware, and handler
- Test Prometheus metrics endpoint

Story 1.4: Error Handling and Error Bus
- Test channel-based error bus creation
- Test error publishing with context
- Test nil error handling
- Test channel full scenario
- Test graceful shutdown
- Fix Close() method to handle multiple calls safely

Story 1.5: HTTP Server and Middleware
- Test server creation with all middleware
- Test request ID middleware
- Test logging middleware
- Test panic recovery middleware
- Test CORS middleware
- Test timeout middleware
- Test health and metrics endpoints
- Test server shutdown

Story 1.6: OpenTelemetry Tracing
- Test tracer initialization (enabled/disabled)
- Test development and production modes
- Test OTLP exporter configuration
- Test graceful shutdown
- Test no-op tracer provider

All tests follow Go testing best practices:
- Table-driven tests where appropriate
- Parallel execution
- Proper mocking of interfaces
- Skip tests requiring external dependencies in short mode

internal/infra/database/client_test.go

@@ -0,0 +1,160 @@
+package database
+
+import (
+	"context"
+	"testing"
+	"time"
+)
+
+func TestNewClient_InvalidDSN(t *testing.T) {
+	t.Parallel()
+
+	cfg := Config{
+		DSN:            "invalid-dsn",
+		MaxConnections: 10,
+		MaxIdleConns:   5,
+	}
+
+	client, err := NewClient(cfg)
+	if err == nil {
+		if client != nil {
+			_ = client.Close()
+		}
+		t.Error("Expected error for invalid DSN, got nil")
+	}
+}
+
+func TestNewClient_ValidConfig(t *testing.T) {
+	t.Parallel()
+
+	// This test requires a real database connection
+	// Skip if DSN is not set
+	dsn := "postgres://goplt:goplt_password@localhost:5432/goplt?sslmode=disable"
+	if testing.Short() {
+		t.Skip("Skipping database test in short mode")
+	}
+
+	cfg := Config{
+		DSN:             dsn,
+		MaxConnections:  10,
+		MaxIdleConns:    5,
+		ConnMaxLifetime: 5 * time.Minute,
+		ConnMaxIdleTime: 10 * time.Minute,
+	}
+
+	client, err := NewClient(cfg)
+	if err != nil {
+		t.Skipf("Skipping test - database not available: %v", err)
+	}
+	defer func() {
+		if err := client.Close(); err != nil {
+			t.Logf("Failed to close client: %v", err)
+		}
+	}()
+
+	if client == nil {
+		t.Fatal("Expected client, got nil")
+	}
+
+	if client.Client == nil {
+		t.Error("Expected Ent client, got nil")
+	}
+
+	if client.db == nil {
+		t.Error("Expected sql.DB, got nil")
+	}
+}
+
+func TestClient_Ping(t *testing.T) {
+	t.Parallel()
+
+	dsn := "postgres://goplt:goplt_password@localhost:5432/goplt?sslmode=disable"
+	if testing.Short() {
+		t.Skip("Skipping database test in short mode")
+	}
+
+	cfg := Config{
+		DSN:            dsn,
+		MaxConnections: 10,
+		MaxIdleConns:   5,
+	}
+
+	client, err := NewClient(cfg)
+	if err != nil {
+		t.Skipf("Skipping test - database not available: %v", err)
+	}
+	defer func() {
+		if err := client.Close(); err != nil {
+			t.Logf("Failed to close client: %v", err)
+		}
+	}()
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	if err := client.Ping(ctx); err != nil {
+		t.Errorf("Ping failed: %v", err)
+	}
+}
+
+func TestClient_Close(t *testing.T) {
+	t.Parallel()
+
+	dsn := "postgres://goplt:goplt_password@localhost:5432/goplt?sslmode=disable"
+	if testing.Short() {
+		t.Skip("Skipping database test in short mode")
+	}
+
+	cfg := Config{
+		DSN:            dsn,
+		MaxConnections: 10,
+		MaxIdleConns:   5,
+	}
+
+	client, err := NewClient(cfg)
+	if err != nil {
+		t.Skipf("Skipping test - database not available: %v", err)
+	}
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	if err := client.Close(); err != nil {
+		t.Errorf("Close failed: %v", err)
+	}
+
+	// Ping should fail after close
+	if err := client.Ping(ctx); err == nil {
+		t.Error("Expected Ping to fail after Close, got nil error")
+	}
+}
+
+func TestClient_DB(t *testing.T) {
+	t.Parallel()
+
+	dsn := "postgres://goplt:goplt_password@localhost:5432/goplt?sslmode=disable"
+	if testing.Short() {
+		t.Skip("Skipping database test in short mode")
+	}
+
+	cfg := Config{
+		DSN:            dsn,
+		MaxConnections: 10,
+		MaxIdleConns:   5,
+	}
+
+	client, err := NewClient(cfg)
+	if err != nil {
+		t.Skipf("Skipping test - database not available: %v", err)
+	}
+	defer func() {
+		if err := client.Close(); err != nil {
+			t.Logf("Failed to close client: %v", err)
+		}
+	}()
+
+	db := client.DB()
+	if db == nil {
+		t.Error("Expected sql.DB from DB(), got nil")
+	}
+}